Is 'let me know once done' OK?

English as a Second Language English Vocabulary, Grammar and Idioms
1

Can someone please tell me if the following sentence is OK?

  1. Please finish the work before noon and let me know once done.
2

Hello d99,

It’s comprehensible, but I wouldn’t recommend it: the drastic elision of “it’s” might surprise your addressee.

Best wishes,

MrP